1. Field of the Invention
The present invention relates to an exposure technique for exposing a substrate to light with a gap between a projection optical system and the substrate filled with liquid.
2. Description of the Related Art
A process for manufacturing a semiconductor device formed from a micropattern of an LSI or VLSI adopts a reduction projection exposure apparatus which reduces a pattern formed on a mask and projects and transfers it onto a substrate coated with a photosensitive agent. As the degree of integration of a semiconductor device increases, further micropatterning becomes necessary. The exposure apparatus has coped with micropatterning along with the development of the resist process.
To increase the resolving power of an exposure apparatus, it is a common practice to shorten the exposure wavelength or to increase the numerical aperture (NA) of a projection optical system.
As for the wavelength of exposure light, a shift, from a 365-nm i-line to KrF excimer laser light having an oscillation wavelength around 248 nm, is in progress, and an ArF excimer laser having an oscillation wavelength around 193 nm is under development. A fluorine (F2) excimer laser having an oscillation wavelength around 157 nm is also under development.
On the other hand, a liquid immersion method or liquid immersion lithography is receiving a great deal of attention as a technique for increasing the resolving power in a completely different way from the above methods. The conventional methods fill, with a gas, the space between the end face of a projection optical system and the surface of an exposure target substrate (e.g., a wafer). However, the liquid immersion method executes projection exposure while filling that space with a liquid. Assume, for example, that the liquid immersion method uses pure water (refractive index: 1.44) as the liquid to be supplied to the space between the projection optical system and the wafer, and sets the maximum incident angle of a light beam imaged on the wafer equal to that in the conventional methods. In this case, the liquid immersion method can advantageously attain a resolving power 1.44 times that in the conventional methods even by using a light source having the same wavelength. This amounts to increasing the NA of the projection optical system in the conventional methods to 1.44 times. The liquid immersion method can attain a resolving power whose NA is one or more, which is practically impossible in the conventional methods.
An attempt to apply the liquid immersion method to an exposure apparatus is recently in progress. For example, Japanese Patent Application Laid-Open No. 6-124873 discloses a prior art exposure apparatus to which the liquid immersion method is applied. FIG. 17A is a side view showing the structure of the exposure apparatus disclosed in Japanese Patent Application Laid-Open No. 6-124873. Referring to FIG. 17A, filling a liquid tank (chamber) 109 with a liquid 130 provides the ability to fill, with the liquid 130, the space between a wafer 102 and the end face (an optical element 107) of a projection optical system 104. The liquid tank 109 accommodates a wafer transfer device, wafer chuck 112, X-Y stage 113, fine moving stage 114, and part of or all of each of coarse alignment devices 111-1 to 111-4. The wafer transfer device loads the wafer 102 from a wafer cassette 110 and sets it on the wafer chuck 112.
Reference numeral 115 denotes a laser interferometer. A reference mirror 116 is attached to the surface of the fine moving stage 114 along the X and Y directions (the Y direction is not shown), and reflects light from the laser interferometer 115 to measure the position of the fine moving stage 114. The liquid tank 109 has a window 117, which passes light from the laser interferometer 115. A heat-insulating material 118 is formed outside the liquid tank 109 and thermally insulates the liquid tank 109 from outside.
As shown in FIG. 17B, Japanese Patent Application Laid-Open No. 6-124873 describes another arrangement example of the exposure apparatus designed to mount the liquid tank 109 on the fine moving stage 114 while placing, in the liquid tank 109, only constituent parts, including the wafer chuck 112, or while directly placing the wafer chuck 112 on the bottom surface of the liquid tank 109. That is, Japanese Patent Application Laid-Open No. 6-124873 describes an exposure apparatus using a method of mounting, in the liquid tank 109, the whole wafer 102 and the end face of the projection optical system 104.
International Publication No. WO99/049504 describes still another example in which the liquid immersion method is applied to an exposure apparatus. International Publication No. WO99/049504 describes an exposure apparatus using a method of supplying a liquid to only the space between a projection optical system and the wafer surface, so as to fill that space.
This method uses a laser interferometer to measure the position of a stage. However, the laser interferometer suffers from a measurement error, due to a change in refractive index of a gas (to be referred to as an ambient gas hereinafter) on its measurement optical path, along with changes in its composition, temperature, and humidity.
For example, an amount Δn of a change in refractive index due to a change in humidity of the ambient gas is expressed by Δn=Ke·Δe where Ke is a coefficient, and Δe (%) is the amount of change in humidity. The coefficient Ke is a constant, which depends on the type of ambient gas used, and the wavelength of a laser used. If, for example, the ambient gas is air, and the light source uses a He—Ne laser, Ke=1E-8 approximately holds. Hence, when the humidity of the ambient gas has changed by 1%, the amount of change in refractive index satisfies:Δn=1E-8*1=1E-8.At this time, if the length of the measurement optical path of the laser interferometer is 1 m, a measurement error ΔL is:ΔL=1E-8*1=10 nm.
Accordingly, it is necessary for an exposure apparatus, to which the conventional liquid immersion method is applied, that a change in refractive index of an ambient gas never influences the measurement accuracy of the laser interferometer. Additionally, to reduce a change in refractive index of the ambient gas on the measurement optical path of the laser interferometer, that exposure apparatus seeks to reduce changes in temperature and humidity of the ambient gas on the measurement optical path of the laser interferometer.
For example, the exposure apparatus in Japanese Patent Application Laid-Open No. 6-124873, having the structure shown in FIG. 17A, provides the ability to prevent the influence of a change in refractive index of an ambient gas on the measurement accuracy of the laser interferometer 115, by directly attaching the laser interferometer 115 to the side surface of the liquid tank 109, as shown in FIG. 17C.
Japanese Patent Application Laid-Open No. 10-303114 exemplifies a technique for reducing changes in temperature and humidity of an ambient gas on the measurement optical path of a laser interferometer. FIG. 18 is a side view showing the structure of an exposure apparatus disclosed in Japanese Patent Application Laid-Open No. 10-303114. At a predetermined height, a wall portion LB is formed on the entire outer peripheral portion of a holder table WH which holds a wafer W. The inner side of the wall portion LB is filled with a liquid LQ at a predetermined depth.
For an exposure scheme, which causes the laser interferometer to project a reference beam BSr onto a reference mirror ML of a projection lens system PL, Japanese Patent Application Laid-Open No. 10-303114 describes a technique for preventing a fluctuation in the optical path of the reference beam BSr by interposing a cover plate 287 between the optical path of the reference beam BSr and the liquid LQ, so as to cut off the flow of the vapor ascending from the liquid LQ.
As described above, an exposure apparatus to which the liquid immersion method is applied seeks to reduce changes in temperature and humidity of a gas on the measurement optical path or to prevent a change in refractive index of the ambient gas from influencing the measurement accuracy of the laser interferometer.
However, it is necessary for the exposure apparatus disclosed in Japanese Patent Application Laid-Open No. 6-124873 to directly attach the laser interferometer 115 to the side surface of the liquid tank 109, as shown in FIG. 17C, in order to prevent a change in refractive index of the ambient gas from influencing the measurement accuracy of the laser interferometer. It is also necessary to place, in the liquid tank 109, the wafer 102, wafer chuck 112, X-Y stage 113, fine moving stage 114, and part of or all of each of coarse alignment devices 111-1 to 111-4. This complicates the apparatus and increases its size, resulting in an increase in its cost.
Additionally, as shown in FIG. 17B, when the wafer 102 and wafer chuck 112 alone are placed in the liquid tank 109, the vapor produced from the liquid 130 changes the composition of an ambient gas on the measurement optical path of the laser interferometer 115. This changes the refractive index of the ambient gas, resulting in degradation in measurement accuracy of the laser interferometer 115. Especially, if the liquid 130 is pure water, not only does the composition of the ambient gas, but also, the humidity of the ambient gas, greatly changes. This causes a larger change in refractive index, so the laser interferometer 115 suffers from a measurement error that is absolutely unallowable.
For the sake of simplicity, the expression that the refractive index of an ambient gas on the measurement optical path of a laser interferometer actually changes indicates the following phenomenon. That is, the refractive index of an ambient gas on the measurement optical path of a laser interferometer changes due to a change in composition and/or humidity of the ambient gas. The expression that the vapor of a liquid from the space between a projection optical system and the wafer surface influences an ambient gas on the measurement optical path of a laser interferometer actually indicates the following phenomenon. That is, the vapor of a liquid from the space between a projection optical system and the wafer surface leaks out onto the measurement optical path of a laser interferometer and, therefore, the refractive index of an ambient gas on the measurement optical path changes.
The exposure apparatus discussed in international Publication WO99/049504 does not consider any vapor of the liquid from the space between the projection optical system and the wafer surface. Therefore, the vapor produced from the liquid from the space between the projection optical system and the wafer surface changes the refractive index of the ambient gas on the measurement optical path of the laser interferometer. The laser interferometer suffers from a measurement error. Consequently, the apparatus often manufactures a defective semiconductor element, resulting in degradation in its productivity.
Since the exposure apparatus in Japanese Patent Application Laid-Open No. 10-303114 mounts the cover plate 287, it is possible to prevent a fluctuation in the optical path of the reference beam BSr. However, the vapor of the liquid LQ still influences a measurement beam BSm, so the fundamental problem remains.